The steam generators of pressurized-water nuclear reactors comprise an outer casing of generally cylindrical shape which contains a bundle of tubes bent in the form of a U and embedded at their ends in a tube plate constituting the exchange part of the bundle. The tube plate is connected to the outer casing and separates the internal volume of the steam generator into a primary part delimited by a water box of generally hemispherical shape located underneath the tube plate and communicating with the internal volume of the tubes of the bundle and a secondary part comprising the internal volume of the outer casing which is located above the tube plate and in which the bundle of tubes is arranged.
A first exchange fluid consisting of the primary water ensuring the cooling of the reactor core is introduced into a first part of the water box, circulates inside the tubes of the bundle and is then recovered in a second part of the water box, before being discharged outside the steam generator and then reintroduced into the vessel of the nuclear reactor.
A second exchange fluid consisting of the feed water is introduced into the secondary part of the steam generator, in which this feed water comes into thermal contact with the primary fluid via the walls of the tubes of the bundle. The thermal exchange with the primary fluid ensures the heating and evaporation of the feed water, and the steam produced is dried and then discharged at the upper end of the secondary part of the steam generator.
After the steam generator has been in operation for some time, it may be necessary to conduct checks, maintenance operations or repairs in the secondary part of the steam generator, for example between the tubes of the bundle.
The bundle of tubes in the steam generator is arranged inside a bundle casing placed coaxially within the outer casing of the steam generator substantially over the entire height of the bundle and comprising a lower end located at some distance above the tube plate of the steam generator.
The zone of the secondary part of the steam generator in which the bundle is arranged is therefore accessible by way of the lower end of the bundle casing, an inspection or attendance device being introduced into the space provided between the lower end of the bundle casing and the tube plate.
In order to conduct inspections or attendance work in the entire zone of the secondary part of the steam generator containing the bundle, the inspection device or attendance tool must be displaceable particularly in circumferential directions in relation to the outer casing and to the bundle casing.
The bundle casing has a diameter substantially smaller than the inside diameter of the outer casing, and therefore an annular space remains between these two casings over the entire height of the bundle. This annular space opens into the inner part of the casing of the bundle in its lower part.
Inspection of the secondary part of the steam generator may involve inspection of the outer surface of the tubes by means of a televisual device, in order to evaluate their appearance and state, after the steam generator has been in operation for a particular time, in the course of which some damage to the tubes of the bundle may have occurred, for example under the effect of corrosion by the feed water or as a result of contact with foreign bodies.
The attendance work in the steam generator may involve extracting foreign bodies which have lodged between the tubes of the bundle.
To date, no simple device has been known for displacing televisual inspection or attendance means in the lower secondary part of a steam generator containing the tube bundle of the generator.